Terminal device, wireless transmission method, base station device, and channel estimation method

ABSTRACT

The present invention adopts a configuration such that when cooperative reception by a plurality of base stations is not applied, a reference signal sequence determined from a selection baseline value corresponding to the number of a sequence group allocated to a cell belonging to the device in question is selected from among a plurality of selection baseline values as a reference signal sequence for non-cooperative reception, whereas when cooperative reception by a plurality of base stations is applied, a reference signal sequence determined from one or more intermediate selection baseline values set between two adjacent selection baseline values corresponding to the number of a sequence group allocated individually to a terminal device is selected among the plurality of selection baseline values as a reference signal sequence for cooperative reception differing from the reference signal sequence for non-cooperative reception.

TECHNICAL FIELD

The present invention relates to a terminal apparatus and a radiotransmission method for transmitting a reference signal, and a basestation apparatus and a channel estimation method for performing channelestimation using a reference signal.

BACKGROUND ART

3GPP LTE (3rd Generation Partnership Project Long-term Evolution) adoptsa ZC (Zadoff-Chu) sequence as a signal sequence used for a referencesignal for data demodulation (DMRS: DeModulation-Reference Signal) usedon an uplink. The ZC sequence is used when the transmission bandwidth is3 RBs (resource blocks) or more.

On an LTE uplink, many DMRS sequences are divided into 30 sequencegroups in each transmission bandwidth (1 to 110 RBs). In each sequencegroup, as shown in FIG. 1, a transmission bandwidth (more specifically,the number of RBs allocated) and a DMRS sequence are associated witheach other. Respective sequence groups are assigned different numbers(sequence group number u=0 to 29), and as shown in FIG. 2, each cell isassigned one sequence group from among #0 to 29. Such sequence groupassignment is called “cell-specific sequence group assignment” or“cell-specific assignment.” A base station (which may also be called“eNB”) broadcasts cell IDs to terminals (which may also be called “UE(User Equipment)”) in the cell. Since the cell IDs and 30 sequence groupnumbers are uniquely assigned with each other beforehand, terminals inthe cell can know cell-specific sequence group numbers from thebroadcast cell-specific IDs. Even when the transmission bandwidth ischanged, the terminal can identify a DMRS sequence number from only asequence group number. Thus, the cell-specific assignment can reducesignaling of sequence numbers. In the cell-specific assignment,different sequence groups are assigned to nearby cells in order toreduce inter-cell interference.

The ZC sequence is a kind of CAZAC (Constant Amplitude and ZeroAuto-correlation Code) sequence and is expressed by following equation1.

$\begin{matrix}\left( {{Equation}\mspace{14mu} 1} \right) & \; \\{{{x_{q}(m)} = e^{{- j}\frac{\pi\;{{qm}{({m + 1})}}}{N_{ZC}^{RS}}}},{0 \leq m \leq {N_{ZC}^{RS} - 1}}} & \lbrack 1\rbrack\end{matrix}$

In equation 1, N_(ZC) ^(RS) is a sequence length of a ZC sequence, q isa ZC sequence number, and m is an element number of the ZC sequence.Sequence length N_(ZC) ^(RS) is a maximum prime number that does notexceed the number of subcarriers in a transmission bandwidth of DMRS,and (N_(ZC) ^(RS)−1) ZC sequences having good cross-correlationcharacteristics can be generated. ZC sequence number q is calculated byequation 2.[2]q=└q+½┘+v·(−1)^(└2q┘)q=N _(ZC) ^(RS)·(u+1)/31  (Equation 2)

Here, regarding v, v=0 when the transmission bandwidth is 5 RBs or lessand v=1 when the transmission bandwidth is 6 RBs or more. Sequence groupnumber u is an integer of u=0 to 29. Number u is associated with a cellID of a serving cell and all UEs in the cell each use one of sequencesthat belong to a common sequence group.

Equation 2 means calculating a ZC sequence number (when v=0)corresponding to q/N_(ZC) ^(RS), which is a ratio between a ZC sequencenumber and a ZC sequence length closest to “(u+1)/31,” and a ZC sequencenumber (when v=1) corresponding to q/N_(ZC) ^(RS) second closest to“(u+1)/31.” Thus, as ZC sequences of each RB, a plurality of sequenceshaving close q/N_(ZC) ^(RS) values are assigned to the same sequencegroup. In the following description, a value that serves as a referencefor calculating a ZC sequence number such as “(u+1)/31” of equation 2 iscalled “sequence selection reference value.” Here, 31 is a maximum primenumber (minimum ZC sequence length) that does not exceed the number ofsubcarriers (=36) assigned to minimum RBs (=3 RBs) of the sequencegroup. Thus, the sequence selection reference value means a ratiobetween a sequence group number and a minimum ZC sequence length.Furthermore, a ratio between a ZC sequence number that determines a ZCsequence and the ZC sequence length, that is, “q/N_(ZC) ^(RS)” is calleda “sequence determination value.”

Sequences of close ZC sequence q/N_(ZC) ^(RS) have a feature of havingsimilar waveforms and a high cross-correlation between sequences. Thus,a sequence group used in one cell is configured of a combination of ZCsequences having a high cross-correlation between ZC sequences, and theprobability that sequences having a high cross-correlation may be usedin neighboring cells is thereby reduced, making it possible to reduceinterference between neighboring cells (e.g., see PTL 1).

DMRS used in 3GPP LTE is transmitted with a transmission bandwidth whichis an integer multiple of 1 RB consisting of 12 subcarriers. Thus, thesequence length of a ZC sequence which is a prime number does notcoincide with the number of subcarriers corresponding to a transmissionbandwidth of DMRS. Thus, as shown in FIG. 3, a sequence obtained bycopying (called “extension”) the top portion of a ZC sequence having aprime number sequence length to the end portion is used as DMRS to beactually transmitted. For example, as DMRS to be transmitted in 3 RBs(36 subcarriers), a ZC sequence having sequence length N_(ZC) ^(RS) of31 is used and DMRS is generated by copying the first five elements tothe end portion.

In LTE-Advanced, which is an evolved version of LTE, a heterogeneousnetwork (HetNet) using a plurality of base stations providing coverageareas in different sizes is under study to achieve a further capacityimprovement. In the operation of HetNet, a pico cell having lowtransmission power is deployed within a coverage area of a macro cellhaving high transmission power. The macro cell may also be called “HPN(High Power Node).” The pico cell may also be called “LPN (Low PowerNode)” or “low power RRH (Remote Radio Head).”

In LTE-Advanced, application of CoMP (Coordinated multiple pointtransmission and reception) is also under study, which is acommunication scheme in which a plurality of cells (base stations)cooperate to transmit and receive signals to and from a terminal in aHetNet environment. CoMP is mainly intended to improve the throughput ofa user located at a cell edge. For example, in the case of uplink CoMP(UL_CoMP), a plurality of cells (which may also be called “basestations” or “reception points”) cooperate to receive uplink signalsfrom one terminal, and received signals are combined by a plurality ofcells to improve receiving quality.

In UL_CoMP, the introduction of MU-MIMO (Multiple User-Multiple InputMultiple Output) communication is under study to achieve a furthersystem performance improvement effect. MIMO communication is a techniquein which transmitting and receiving sides are provided with a pluralityof antennas to enable different signal sequences to be simultaneouslyand spatially multiplexed at the same frequency. MU-MIMO communicationis a technique in which MIMO communication is carried out by a pluralityof terminals to which UL_CoMP is applied, that is, terminals thatcooperate to receive and combine transmission signals in a plurality ofcells (hereinafter, may also be referred to as “CoMP_UE”) and a basestation. MU-MIMO communication can improve the frequency utilizationefficiency of the system.

In MU-MIMO communication, it is necessary to transmit DMRSs which areorthogonalized among terminals to demultiplex signals of differentterminals. As a method of orthogonalizing DMRSs, a ZC sequence (CS-ZCsequence) may be used in which a different cyclic shift (CS) for eachterminal is applied. Setting a value larger than a maximum propagationdelay time of transmission signals of terminals as a cyclic shift valuemakes it possible to orthogonalize a plurality of CS-ZC sequencesgenerated from ZC sequences of the same sequence group.

However, when UL_CoMP is applied, DMRSs need to be received from aplurality of different cells. For this reason, the aforementionedcell-specific sequence group assignment may cause ZC sequence numbers ofCoMP_UE to differ from each other, making it impossible to orthogonalizeCS-ZC sequences to be used as DMRSs.

Thus, as shown in FIG. 4, studies are being carried out on thepossibility of introducing UE-specific sequences (sequence group #17 inFIG. 4) in which sequences are indicated individually for the respectiveterminals instead of cell-specific sequences (sequence groups #1 and #2in FIG. 4) for CoMP_UE. Assigning the same sequence group to terminalsthat perform CoMP reception allows DMRSs to be orthogonalized amongCoMP_UEs.

CITATION LIST Patent Literature

PTL 1

-   Japanese Patent Publication No. 4624475

Non-Patent Literature

NPL 1

-   3GPP TS36.211 V10.2.0, “Physical Channels and Modulation (Release    10),” 5.5 Reference Signals, June 2011

SUMMARY OF INVENTION Technical Problem

However, when a certain sequence group is selected from existingsequence groups and assigned to CoMP_UE, there is a problem in that,even when a sequence group which is not used in the neighborhood isassigned, a sequence group reuse distance is reduced. The term “reusedistance” refers to a spatial distance from one cell to the nearest cellto which the same sequence group is applied. When the reuse distancebecomes smaller, the inter-cell interference between DMRSs used inCoMP_UE and Non-CoMP_UE increases. This situation is shown in FIG. 5 andFIG. 6.

As shown in FIG. 5, when UL_CoMP is not applied, a different sequencegroup is applied to each plurality of cells (base stations) that formone cell cluster (cell-specific sequence assignment) as described above.Thus, the reuse distance between sequence groups increases.

In contrast, as shown in FIG. 6, when UL_CoMP is applied, the samesequence group is applied to a plurality of cells that cooperate toreceive transmission signals of CoMP_UE (UE-specific sequenceassignment).

FIG. 6 illustrates an example in which sequence group #17 isUE-specifically assigned to UE located on a boundary between cells usingsequence groups #1 and #3 through cell-specific assignment in order toapply UL_CoMP. In this case, the distance (sequence group reusedistance) between a UE using sequence group #17 through UE-specificassignment, that is, a UE located on a boundary between cells usingsequence groups #1 and #3 and a cell using sequence group #17 throughcell-specific assignment becomes smaller. A decrease in the distancemeans a decrease in distance attenuation of signals. Therefore,inter-cell interference between DMRSs increases between CoMP_UE usingsequence group #17 through UE-specific assignment and non-CoMP_UE usingsequence group #17 through cell-specific assignment.

The problems relating to uplink DMRS, which is one of reference signalsequences, have been described so far. However, such problems also occurwith an uplink sounding reference signal (SRS) which is anotherreference signal sequence.

An object of the present invention is to provide a terminal apparatus, atransmission method, a base station apparatus and a channel estimationmethod make possible UE-specific reference signal sequence assignment,and transmission and reception of radio signals without any increase ininter-cell interference, when CoMP is applied.

Solution to Problem

A terminal apparatus according to an aspect of the present inventionincludes: a reference signal generating section that selects, whencoordinated-reception by a plurality of base stations is not applied tothe terminal apparatus, a reference signal sequence determined from aselection reference value corresponding to a sequence group numberassigned to a cell to which the terminal apparatus belongs among aplurality of selection reference values each being a ratio between avalue corresponding to a sequence group number to which a plurality ofreference signal sequences belong and a sequence length of a referencesignal sequence of a minimum transmission bandwidth in the sequencegroup as a non-coordinated-reception reference signal sequence, thatselects, when coordinated-reception by the plurality of base stations isapplied to the terminal apparatus, a reference signal sequencedetermined from one or a plurality of intermediate selection referencevalues set between two adjacent selection reference values correspondingto a sequence group number assigned specifically to the terminalapparatus among the plurality of selection reference values as acoordinated-reception reference signal sequence different from thenon-coordinated-reception reference signal sequence, and that generatesa reference signal based on the selected reference signal sequence; anda transmitting section that transmits the generated reference signal.

A radio transmission method according to an aspect of the presentinvention includes: selecting, when coordinated-reception by a pluralityof base stations is not applied to a terminal apparatus, a referencesignal sequence determined from a selection reference valuecorresponding to a sequence group number assigned to a cell to which theterminal apparatus belongs among a plurality of selection referencevalues each being a ratio between a value corresponding to a sequencegroup number to which a plurality of reference signal sequences belongand a sequence length of a reference signal sequence of a minimumtransmission bandwidth in the sequence group as anon-coordinated-reception reference signal sequence, and selecting, whencoordinated-reception by the plurality of base stations is applied tothe terminal apparatus, a reference signal sequence determined from oneor a plurality of intermediate selection reference values set betweentwo adjacent selection reference values corresponding to a sequencegroup number assigned specifically to the terminal apparatus among theplurality of selection reference values as a coordinated-receptionreference signal sequence different from the non-coordinated-receptionreference signal sequence; generating a reference signal based on theselected reference signal sequence; and transmitting the generatedreference signal.

A base station apparatus according to an aspect of the present inventionincludes: a setting section that selects, when coordinated-reception bya plurality of base stations is not applied to a terminal apparatus, areference signal sequence determined from a selection reference valuecorresponding to a sequence group number assigned to a cell to which theterminal apparatus belongs among a plurality of selection referencevalues each being a ratio between a value corresponding to a sequencegroup number to which a plurality of reference signal sequences belongand a sequence length of a reference signal sequence of a minimumtransmission bandwidth in the sequence group as anon-coordinated-reception reference signal sequence, and that selects,when the coordinated-reception by the plurality of base stations isapplied to the terminal apparatus, a reference signal sequencedetermined from one or a plurality of intermediate selection referencevalues set between two adjacent selection reference values correspondingto a sequence group number assigned specifically to the terminalapparatus among the plurality of selection reference values as thecoordinated-reception reference signal sequence different from thenon-coordinated-reception reference signal sequence; a receiving sectionthat receives a signal transmitted from the terminal apparatus; and achannel estimation section that performs channel estimation based on thereceived signal and the reference signal sequence selected by thesetting section.

A channel estimation method according to an aspect of the presentinvention includes: selecting, when coordinated-reception by a pluralityof base stations is not applied to a terminal apparatus, a referencesignal sequence determined from a selection reference valuecorresponding to a sequence group number assigned to a cell to which theterminal apparatus belongs among a plurality of selection referencevalues each being a ratio between a value corresponding to a sequencegroup number to which a plurality of reference signal sequences belongand a sequence length of a reference signal sequence of a minimumtransmission bandwidth in the sequence group as anon-coordinated-reception reference signal sequence, and selecting, whencoordinated-reception by the plurality of base stations is applied tothe terminal apparatus, a reference signal sequence determined from oneor a plurality of intermediate selection reference values set betweentwo adjacent selection reference values corresponding to a sequencegroup number assigned specifically to the terminal apparatus among theplurality of selection reference values as a coordinated-receptionreference signal sequence different from the non-coordinated-receptionreference signal sequence; receiving a signal transmitted from theterminal apparatus; and performing channel estimation based on thereceived signal and the selected reference signal sequence.

Advantageous Effects of Invention

According to the present invention, it is possible to performterminal-specific reference signal sequence assignment, and transmissionand reception of radio signals without any increase in inter-cellinterference, when coordinated-reception is applied.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a table illustrating sequence numbers of ZC sequences assignedto each sequence group in cell-specific sequence assignment;

FIG. 2 illustrates an example of a sequence group assigned to aplurality of cells;

FIG. 3 illustrates extension processing on a ZC sequence to compensatefor a difference between a transmission bandwidth and a sequence length;

FIG. 4 illustrates a situation in which cell-specific sequences of thesame sequence group are assigned to a plurality of terminals to performCoMP;

FIG. 5 illustrates a reuse distance when cell-specific sequenceassignment is adopted in non-CoMP;

FIG. 6 illustrates a reuse distance when UE-specific assignment isadopted in UL_CoMP;

FIG. 7 is a block diagram illustrating main components of terminal 100according to Embodiment 1 of the present invention;

FIG. 8 is a block diagram illustrating a configuration of terminal 100according to Embodiment 1;

FIG. 9 illustrates an example of extension processing on a ZC sequencein accordance with a transmission bandwidth of 6 RBs;

FIG. 10 is a block diagram illustrating main components of base station200 according to Embodiment 1 of the present invention;

FIG. 11 is a block diagram illustrating a configuration of base station200 according to Embodiment 1;

FIG. 12 illustrates selection reference values of a non-CoMP_UE sequencegroup and a CoMP_UE sequence group according to Embodiment 1;

FIG. 13 illustrates a method of selecting a ZC sequence in the case of10 RBs according to Embodiment 1;

FIG. 14 illustrates a variation of selection reference values of aCoMP_UE sequence group according to Embodiment 1;

FIGS. 15A and 15B illustrate results in cases where a ZC sequenceselection method according to Embodiment 1 is applied when thetransmission bandwidth is 3 RBs to 6 RBs;

FIG. 16 illustrates results of selection of ZC sequences according toEmbodiment 2 when the transmission bandwidth is 3 RBs to 6 RBs;

FIG. 17 illustrates a first example of truncation processing on a ZCsequence according to Embodiment 2;

FIG. 18 illustrates a second example of truncation processing on a ZCsequence according to Embodiment 2;

FIG. 19 illustrates a method of deriving a CoMP_UE sequence groupaccording to Embodiment 3;

FIG. 20 illustrates another example of the method of deriving a CoMP_UEsequence group according to Embodiment 3;

FIG. 21 illustrates an example in which sequence group assignmentaccording to the present invention is applied to a system including alow power base station; and

FIG. 22 is a table illustrating a variation of the method of assigning aCoMP_UE ZC sequence.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings.

Embodiment 1

[Overview of Communication System]

A communication system according to Embodiment 1 of the presentinvention includes a transmitting apparatus and a receiving apparatus.Especially, the present embodiment will be described assuming that thetransmitting apparatus is terminal 100 and the receiving apparatus isbase station 200. This communication system is, for example, anLTE-Advanced system. Terminal 100 is, for example, a terminal compliantwith the LTE-Advanced system and base station 200 is, for example, abase station compliant with the LTE-Advanced system. For example, whenterminal 100 operates, for example, as a CoMP_UE which adopts UL_CoMP,signals transmitted from terminal 100 are received by a plurality ofbase station 200 in cooperation.

[Configuration of Terminal 100]

FIG. 7 is a block diagram illustrating main components of terminal 100according to Embodiment 1 of the present invention.

As shown in FIG. 7, terminal 100 according to the present embodiment 1is provided with antenna 101, reference signal generating section 113and transmitting section 105.

Reference signal generating section 113 sets a ZC sequence to be usedfor DMRS first. More specifically, reference signal generating section113 sets a non-CoMP_UE reference signal sequence when CoMP(corresponding to coordinated-reception) is not applied to terminal 100.Of a plurality of selection reference values (u+1)/31, a referencesignal sequence determined from a selection reference valuecorresponding to a sequence group number assigned to the cell to whichterminal 100 belongs is selected as this reference signal sequence. The“plurality of selection reference values (u+1)/31” are ratios betweenvalues u+1 (where, u is 0 to 29) corresponding to each sequence groupnumber to which a plurality of reference signal sequences belong andsequence length 31 of a reference signal sequence of a minimumtransmission bandwidth in the sequence group. When CoMP is applied toterminal 100, reference signal generating section 113 sets a CoMP_UEreference signal sequence. Of the plurality of selection referencevalues (u+1)/31, a reference signal sequence determined from one or aplurality of intermediate selection reference values set between twoadjacent selection reference values corresponding to a sequence groupnumber individually assigned to terminal 100 is selected as thisreference signal sequence. Reference signal generating section 113generates DMRS based on the selected reference signal sequence and sendsthe DMRS to transmitting section 105.

Transmitting section 105 transmits the generated DMRS to base station200 via antenna 101.

FIG. 8 is a block diagram illustrating a configuration of terminal 100according to Embodiment 1.

More specifically, as shown in FIG. 8, terminal 100 includes antenna101, reference signal processing section 110, mapping section 102, IFFT(Inverse Fast Fourier Transform) section 103, CP adding section 104,transmitting section 105, receiving section 106 and demodulation section107. Reference signal processing section 110 includes CoMP determiningsection 111, sequence calculation section 112 and reference signalgenerating section 113.

Of these sections, mapping section 102, IFFT section 103, CP addingsection 104 and transmitting section 105 constitute transmissionprocessing section 108. Receiving section 106 and demodulation section107 constitute reception processing section 109. Next, details of therespective sections will be described.

Receiving section 106 receives a signal transmitted from base station200 via antenna 101, applies reception processing such asdown-conversion, and/or A/D conversion to the received signal andoutputs the received signal subjected to the reception processing todemodulation section 107.

Demodulation section 107 demodulates a control signal included in thereceived signal inputted from receiving section 106 and outputs thedemodulated control information to CoMP determining section 111. Thiscontrol signal includes information indicating whethercoordinated-reception is applied or not, and is transmitted from basestation 200. Demodulation section 107 demodulates received data includedin the received signal inputted from receiving section 106 and sends thereceived data to a baseband section.

CoMP determining section 111 determines, based on the controlinformation indicated from base station 200, whethercoordinated-reception by a plurality of receiving apparatuses (basestations 200) is applied to the terminal (described as “CoMP_UE”) or not(described as “non-CoMP_UE”). CoMP determining section 111 outputs thedetermination result to sequence calculation section 112.

In the communication system of the present embodiment, for example, basestation 200 is assumed to explicitly indicate whether terminal 100 is aCoMP_UE or non-CoMP_UE to terminal 100 beforehand. In this case, CoMPdetermining section 111 can make the above-described determination basedon the indicated information. CoMP determining section 111 may alsoimplicitly determine, from other information set in terminal 100,whether the terminal is CoMP_UE or non-CoMP_UE. As the otherinformation, for example, a UE-specific sequence group number,UE-specific sequence number or virtual cell ID to obtain a sequencegroup number is applicable. In this case, CoMP determining section 111can determine that the terminal is CoMP_UE if the information is set orcan be set and determine that the terminal is non-CoMP_UE if theinformation is not set or cannot be set.

Sequence calculation section 112 calculates a transmission bandwidth(=M^(RS) [subcarriers]) of DMRS indicated from base station 200, ZCsequence length N_(ZC) ^(RS) [subcarriers] depending on whether theterminal is CoMP_UE or non-CoMP_UE, and ZC sequence number q or q′.Sequence calculation section 112 outputs the calculation result toreference signal generating section 113. Here, ZC sequence length N_(ZC)^(RS) is calculated as a maximum prime number which is smaller thanM^(RS). For example, N_(ZC) ^(RS)=71 when M^(RS)=72 (6 RBs). The methodof calculating ZC sequence number q or q′ will be described in detaillater.

Reference signal generating section 113 generates a ZC sequenceaccording to equation 1 based on ZC sequence length N_(ZC) ^(RS) and ZCsequence numbers q and q′ set by sequence calculation section 112.Furthermore, reference signal generating section 113 extends thegenerated ZC sequence of length N_(ZC) ^(RS) to transmission bandwidthM^(RS) to create a DMRS sequence, and outputs the DMRS sequence tomapping section 102. As the extension method as shown in FIG. 9, amethod is adopted whereby, for example, the leading portion of the ZCsequence is copied (extended) to the rear portion. FIG. 9 illustrates anexample in which extension processing is applied to the ZC sequence inaccordance with a transmission bandwidth of 6 RBs.

Mapping section 102 maps the generated DMRS to a band corresponding to atransmission band of terminal 100 and outputs the mapped signal to IFFTsection 103. In addition, mapping section 102 maps the transmission datato a band corresponding to the transmission band of terminal 100 andoutputs the mapped signal to IFFT section 103.

IFFT section 103 applies IFFT processing to the signal inputted frommapping section 102 and outputs the signal subjected to the IFFTprocessing to CP (Cyclic Prefix) adding section 104.

CP adding section 104 adds the same signal as the rear end portion ofthe signal after the IFFT to the leading portion as a CP and outputs thesignal to transmitting section 105.

Transmitting section 105 applies transmission processing such as D/Aconversion, up-conversion, and/or amplification to the signal with theCP and transmits the signal subjected to the transmission processing viaantenna 101. The signal transmitted includes transmission data and DMRS.

[Configuration of Base Station 200]

FIG. 10 is a block diagram illustrating main components of base station200 according to Embodiment 1 of the present invention.

As shown in FIG. 10, base station 200 according to present Embodiment 1is provided with antenna 201, receiving section 202, reference signalsetting section 214 and CH estimation section 210.

Reference signal setting section 214 sets a ZC sequence to be used by aUE for DMRS. More specifically, when CoMP is not applied to the UE,reference signal setting section 214 sets a non-CoMP_UE reference signalsequence in the UE. Of a plurality of selection reference values(u+1)/31, a reference signal sequence determined from a selectionreference value corresponding to a sequence group number assigned to thecell to which terminal 100 belongs is selected as this reference signalsequence. The “plurality of selection reference values (u+1)/31” areratios between values u+1 (where, u is 0 to 29) corresponding to eachsequence group number to which a plurality of reference signal sequencesbelong and sequence length 31 of a reference signal sequence of aminimum transmission bandwidth in the sequence group. When CoMP isapplied to the UE, reference signal setting section 214 sets a CoMP_UEreference signal sequence. Of a plurality of selection reference values(u+1)/31, a reference signal sequence determined from one or a pluralityof intermediate selection reference values set between two adjacentselection reference values corresponding to a sequence group numberindividually assigned to terminal 100 is selected as this referencesignal sequence. Reference signal setting section 214 sends theselection result to CH estimation section 210.

CH estimation section 210 performs channel estimation for coherentdetection from DMRS included in the received signal based on theselection result from reference signal setting section 214.

FIG. 11 is a block diagram illustrating a configuration of base station200 according to Embodiment 1.

More specifically, as shown in FIG. 11, base station 200 is providedwith antenna 201, receiving section 202, CP removing section 203,demultiplexing section 204, FFT sections 205 and 208, demapping sections206 and 209, CH estimation section 210, frequency domain equalizationsection 207, IDFT (Inverse Discrete Fourier Transform) section 211,demodulation section 212, decoding section 213, reference signal settingsection 214, modulation section 215 and transmitting section 216.

Of these sections, modulation section 215 and transmitting section 216constitute transmission processing section 217. CP removing section 203,demultiplexing section 204, FFT (Fast Fourier transform) sections 205and 208, demapping sections 206 and 209, frequency domain equalizationsection 207, IDFT section 211, demodulation section 212 and decodingsection 213 constitute reception processing section 218. Referencesignal setting section 214 includes a sequence calculation section (notshown). Next, details of the sections will be described.

Reference signal setting section 214 has substantially the sameconfiguration as that of reference signal processing section 110 (FIG.8) of terminal 100. Reference signal setting section 214 determineswhether terminal 100 is a CoMP_UE or not, generates a DMRS sequencecorresponding to CoMP_UE or non-CoMP_UE (the same sequence as the DMRSsequence transmitted by terminal 100) and outputs the DMRS sequence toCH estimation section 210. Reference signal setting section 214 outputscontrol information to implicitly or explicitly inform terminal 100 ofwhether terminal 100 is CoMP_UE or non-CoMP_UE to modulation section215.

Modulation section 215 modulates the control information andtransmission data outputted from reference signal setting section 214and outputs the modulated signal to transmitting section 216.

Transmitting section 216 applies transmission processing such as D/Aconversion, up-conversion, and/or amplification to the signal outputtedfrom the modulation section and transmits the signal subjected to thetransmission processing from antenna 201.

Receiving section 202 applies reception processing such asdown-conversion, A/D conversion to a signal received via antenna 201 andoutputs the signal subjected to the reception processing to CP removingsection 203. The received signal includes a data signal and DMRS.

CP removing section 203 removes a CP from the signal subjected to thereception processing and outputs the signal without the CP todemultiplexing section 204.

Demultiplexing section 204 demultiplexes the signal inputted from CPremoving section 203 into a DMRS and data signal. Demultiplexing section204 outputs the data signal to FFT section 205 and outputs the DMRS toother FFT section 208.

FFT section 208 on the DMRS side applies FFT processing to the DMRSinputted from demultiplexing section 204; to transform the DMRS from atime domain signal into a frequency domain signal. FFT section 208outputs the DMRS transformed into the frequency domain signal todemapping section 209.

Demapping section 209 extracts a portion corresponding to a transmissionband of each terminal 100 from the DMRS in the frequency domain inputtedfrom FFT section 208 and outputs the extracted DMRS to CH estimationsection 210.

CH estimation section 210 performs channel estimation (calculation of achannel estimate value) for synchronization detection of an uplinkphysical channel using the DMRS. To be more specific, CH estimationsection 210 divides the DMRS inputted from demapping section 209 by theDMRS sequence inputted from reference signal setting section 214 firstand applies IFFT processing to the division result (correlation value).Next, CH estimation section 210 applies mask processing to the signalsubjected to the IFFT processing, and thereby extracts a correlationvalue of a section in which a correlation value of a desired cyclicshift sequence exists (window portion). Next, CH estimation section 210applies DFT (Discrete Fourier Transform) processing to the extractedcorrelation value and outputs the correlation value subjected to the DFTprocessing to frequency domain equalization section 207 as a channelestimate value. Here, the outputted signal is a signal representing afrequency fluctuation of a propagation path (frequency response of apropagation path).

FFT section 205 on the data side applies FFT processing to the datasignal inputted from demultiplexing section 204 to transform the datasignal from a time domain signal into a frequency domain signal. FFTsection 205 outputs the data signal transformed into the frequencydomain signal to demapping section 206.

Demapping section 206 extracts a data signal of a portion correspondingto the transmission band of each terminal from the signal inputted fromthe FFT section and outputs each extracted signal to frequency domainequalization section 207.

Frequency domain equalization section 207 applies equalizationprocessing to the data signal inputted from demapping section 206 usingthe signal inputted from CH estimation section 210 (frequency responseof the propagation path) and outputs the signal subjected to theequalization processing to IDFT section 211.

IDFT section 211 applies IDFT processing to the data signal inputtedfrom frequency domain equalization section 207 and transforms thefrequency domain signal back into a time domain signal. IDFT section 211outputs the time domain signal to demodulation section 212.

Demodulation section 212 applies demodulation processing to the signalinputted from the IDFT section and outputs the signal subjected to thedemodulation processing to decoding section 213.

Decoding section 213 applies decoding processing to the signal inputtedfrom demodulation section 212 and extracts received data.

[Calculation Processing on Sequence Numbers q and q′]

Here, calculation processing on sequence numbers q and q′ performed bysequence calculation section 112 of terminal 100 and the sequencecalculation section of base station 200 will be described in detail.

FIG. 12 illustrates selection reference values of a non-CoMP_UE sequencegroup and a CoMP_UE sequence group according to Embodiment 1. FIG. 13illustrates a method of selecting a ZC sequence in the case of 10 RBsaccording to Embodiment 1.

When terminal 100 is a non-CoMP_UE, sequence calculation section 112 ofterminal 100 calculates non-CoMP_UE sequence selection reference valueB_(u)=(u+1)/31 using sequence group number u as shown in equation 3first. Here, a cell-specific sequence group number u (one of numbers 0to 29) assigned to a serving cell is applied as the sequence groupnumber u.

On the other hand, when terminal 100 is a CoMP_UE, sequence calculationsection 112 of terminal 100 calculates CoMP_UE sequence selectionreference value B_(u)′, which is different from any non-CoMP_UE sequenceselection reference value B_(u) using the sequence group number u asshown in equation 4. The explicitly or implicitly indicated UE-specificsequence group number u (e.g., one of numbers 0 to 29) is applied as thesequence group number u used here.

As shown in FIG. 12, a predetermined number of CoMP_UE sequenceselection reference values B_(u)′ (e.g., one in the case of FIG. 12) areset between adjacent sequence selection reference values B_(u) of thenon-CoMP_UE sequence group. This CoMP_UE sequence selection referencevalue B_(u)′ corresponds to an intermediate sequence selection referencevalue. Equation 4 is a calculation expression when one CoMP_UE sequenceselection reference value B_(u)′ is set between adjacent sequenceselection reference values B_(u) of the non-CoMP_UE sequence group.

Next, sequence calculation section 112 of terminal 100 calculatessequence number q corresponding to sequence length N_(ZC) ^(RS) based onnon-CoMP_UE sequence selection reference value B_(u) or CoMP_UE sequenceselection reference value B_(u)′. FIG. 13 schematically illustrates thiscalculation processing.

Although only some of numerical value intervals are shown in FIG. 13, inthe case of a sequence for 10 RBs (sequence length N_(ZC) ^(RS)=113), anumber of sequence determination values “q/N_(ZC) ^(RS) (or q′/N_(ZC)^(RS))=1/113 to 112/113” corresponding to the sequence length uniformlyappear in the numerical value interval of “0 to 1.” In addition, 30non-CoMP_UE sequence selection reference values “B_(u)=1/31 to 30/31”and 30 CoMP_UE sequence selection reference values “B_(u)′=1/62, 3/62,5/62, . . . , 61/62” are set in this numerical value section. Thenumerical value section between sequence selection reference value B_(u)of the non-CoMP_UE sequence group (interval of #0 to 1 in the case ofFIG. 13) corresponds to a sequence selection numerical value interval.

When terminal 100 is a non-CoMP_UE, sequence calculation section 112 ofterminal 100 calculates non-CoMP_UE ZC sequence number q usingpreviously calculated sequence selection reference value B_(u) as shownin equation 5-1.

Equation 5-1 corresponds to calculating a ZC sequence number (when v=0)and a ZC sequence number (when v=1) where the absolute value of thedifference between the sequence determination value (q/N_(ZC) ^(RS)) andnon-CoMP_UE sequence selection reference value B_(u) is closest to 0 andsecond closest to 0, respectively.

Here, for example, let us suppose that the transmission bandwidth is 10RBs and non-CoMP_UE sequence selection reference value B_(u) is “1/31.”In this case, as shown in FIG. 13, ZC sequence numbers “q=3 and 4” arecalculated so that two sequence determination values “q/N_(ZC)^(RS)=3/113 and 4/113” closest to non-CoMP_UE sequence selectionreference values “B_(u)=1/31” are selected. Moreover, if non-CoMP_UEsequence selection reference value B_(u) is “2/31,” ZC sequence numbers“q=7 an 8” are calculated so that two sequence determination values“q/N_(ZC) ^(RS)=7/113 and 8/113” closest to this sequence selectionreference value B_(u) are selected.

On the other hand, when terminal 100 is a CoMP_UE, sequence calculationsection 112 of terminal 100 calculates CoMP_UE ZC sequence number q′using previously calculated CoMP_UE sequence selection reference valueB_(u)′ as shown in equation 5-2.

Equation 5-2 corresponds to calculating a ZC sequence number (when v=0)and ZC sequence number (when v=1) where the absolute value of thedifference between sequence determination value (q′/N_(ZC) ^(RS)) andCoMP_UE sequence selection reference value B_(u)′ is closest to 0 andsecond closest to 0 respectively.

Here, for example, let us suppose that the transmission bandwidth is 10RBs and CoMP_UE sequence selection reference value B_(u)′ is “3/62.” Inthis case, as shown in FIG. 13, ZC sequence numbers “q′=5 and 6” arecalculated so that two sequence determination values “q′/N_(ZC)^(RS)=5/113 and 6/113” closest to CoMP_UE sequence selection referencevalue “B_(u)′=3/62” are selected.

$\begin{matrix}{{\left( {{Equation}\mspace{14mu} 3} \right){{B_{u} = \frac{u + 1}{31}},\left( {{u = 0},\ldots\mspace{14mu},29} \right)}\left( {{Equation}\mspace{14mu} 4} \right){{B_{u}^{\prime} = {{\frac{u}{31} + \frac{1}{62}} = \frac{{2u} + 1}{62}}},\left( {{u = 0},\ldots\mspace{14mu},29} \right)}\left( {{Equation}\mspace{20mu} 5\text{-}1} \right)q = {\left\lfloor {\overset{\_}{q} + {1/2}} \right\rfloor + {v \cdot \left( {- 1} \right)^{\lfloor{2\overset{\_}{q}}\rfloor}}}}{\overset{\_}{q} = {N_{ZC}^{RS} \cdot {B_{u}\left( {{Equation}\mspace{14mu} 5\text{-}2} \right)}}}{q^{\prime} = {\left\lfloor {{\overset{\_}{q}}^{\prime} + {1/2}} \right\rfloor + {v \cdot \left( {- 1} \right)^{\lfloor{2{\overset{\_}{q}}^{\prime}}\rfloor}}}}{{\overset{\_}{q}}^{\prime} = {N_{ZC}^{RS} \cdot B_{u}^{\prime}}}} & \lbrack 3\rbrack\end{matrix}$

Thus, in the present embodiment, different values are used as referencevalues to select a sequence for non-CoMP_UE sequence selection referencevalue B_(u) and CoMP_UE sequence selection reference value B_(e)′. Thus,it is possible to generate, for CoMP_UE, a ZC sequence of the ZCsequence group having a correlation characteristic different from theconventional 30 non-CoMP_UE sequence groups. Using such a ZC sequenceprevents a sequence having a high cross-correlation from being used in anearby cell, and can thereby reduce inter-cell interference.

In the present embodiment, as CoMP_UE sequence selection referencevalues B_(u)′, such values are adopted that sequence selection referencevalues B_(u) and B_(u)′ are deployed at uniform intervals between twoadjacent non-CoMP_UE sequence selection reference values B_(u).Therefore, when a sequence determination value (ratio between a ZCsequence number and a sequence length) is selected based on thesesequence selection reference values B_(u) and B_(u)′, it is possible tospace the differences in sequence determination values among a pluralityof sequence groups substantially uniformly.

As described above, a cross-correlation value between different ZCsequences increases as the ratios between a ZC sequence number and asequence length are closer to each other (see NPL 1). Conversely, thecross-correlation value decreases as the ratios between a ZC sequencenumber and a sequence length are more distant from each other. Thus, bygenerating sequence groups as described above, it is possible to avoidincreases in inter-cell interference of DMRS used for non-CoMP_UE andCoMP_UE.

The sequence calculation section of base station 200 calculates CoMP_UEsequence number q and non-CoMP_UE sequence number q′ through the sameprocessing as that described above based on whether terminal 100 as thecommunicating party is a CoMP_UE or non-CoMP_UE.

Note that similar effects can also be achieved using equation 4′ insteadof equation 4.

$\begin{matrix}\left( {{Equation}\mspace{20mu} 4} \right)^{\prime} & \; \\{{B_{u}^{\prime} = {{\frac{u + 1}{31} + \frac{1}{62}} = \frac{{2u} + 3}{62}}},\left( {{u = 0},\ldots\mspace{14mu},29} \right)} & \lbrack 4\rbrack\end{matrix}$

CoMP_UE sequence selection reference value B_(u)′ in equation 4corresponds to 30 new sequence selection reference values obtained byshifting 30 non-CoMP_UE sequence selection reference value B_(u) by 1/62to the negative side. In contrast, CoMP_UE sequence selection referencevalue B_(u)′ of equation 4′ corresponds to 30 new sequence selectionreference values obtained by shifting 30 non-CoMP_UE sequence selectionreference value B_(u) by 1/62 to the positive side.

In the example of FIG. 12, one CoMP_UE sequence selection referencevalue B_(u)′ is provided between adjacent non-CoMP_UE sequence selectionreference values B_(u), but a plurality of (such as two or three)CoMP_UE sequence selection reference values B_(u)′ may be provided toset more CoMP_UE sequence selection reference values B_(u)′. This makesit possible to generate more CoMP_UE sequence groups than non-CoMP_UEsequence groups. Next, an example of this case will be described.

FIG. 14 illustrates a modification of selection reference values ofCoMP_UE sequence groups.

A case will be described here where Y (Y corresponds to the number ofdivisions of a reference value to select a sequence) CoMP_UE sequencegroups are generated between non-CoMP_UE sequence selection referencevalues (equation 3). In this case, Y CoMP_UE sequence selectionreference values B_(u,y)′ of CoMP_UE sequence group (u, y) (where, y=0,. . . , Y−1) are set between adjacent sequence selection referencevalues B_(u) of the non-CoMP_UE sequence group. More specifically,sequence selection reference values B_(u, y)′ of the CoMP_UE sequencegroup (u, y) are calculated according to equation 6 or equation 6′.Equations 4 and 4′ are equivalent to equations 6 and 6′ when Y=1. Theexample of FIG. 14 illustrates selection reference values of a sequencegroup when Y=2.

$\begin{matrix}{\left( {{Equation}\mspace{14mu} 6} \right){{B_{u,y}^{\prime} = {\frac{u}{31} + \frac{y + 1}{31 \cdot \left( {Y + 1} \right)}}},\left( {{u = 0},\ldots\mspace{14mu},{29;{y = 0}},{\ldots\mspace{14mu}\left( {Y - 1} \right)}} \right)}\left( {{Equation}\mspace{14mu} 6} \right)^{\prime}{{B_{u,y}^{\prime} = {\frac{u + 1}{31} + \frac{y + 1}{31 \cdot \left( {Y + 1} \right)}}},\left( {{u = 0},\ldots\mspace{14mu},{29;{y = 0}},{\ldots\mspace{14mu}\left( {Y - 1} \right)}} \right)}} & \lbrack 5\rbrack\end{matrix}$

By generating a CoMP_UE sequence group in this way, it is possible touniformly space the ratios between a ZC sequence number and a sequencelength between CoMP_UE and non-CoMP_UE sequence groups, and therebyreduce inter-cell interference.

A configuration has been described above as an example where CoMP_UEsequence selection reference values B_(u)′ and B_(u, y)′ are set betweentwo adjacent non-CoMP_UE sequence selection reference values B_(u) atuniform intervals. However, CoMP_UE sequence selection reference valuesB_(u)′ and B_(u, y)′ may also be set to values slightly different fromthe values set at the above-described uniform intervals. Although theyare set to slightly different values, it is possible to assign sequencegroups just as in the case where the values are set at uniformintervals.

When fewer CoMP_UE sequence groups than non-CoMP_UE sequence groups aregenerated, a maximum number of CoMP_UE sequence group number u may belimited using the same equation.

As described above, according to present Embodiment 1, sequences withratios between a ZC sequence number and a ZC sequence length (differentcorrelation characteristics) different from those of non-CoMP_UE DMRS ZCsequences are used as CoMP_UE DMRS ZC sequences. Therefore, a reusedistance which is a distance between two cells in which the samesequence group is used will not decrease. It is thereby possible toprevent inter-cell interference of DMRS used for CoMP_UE and non-CoMP_UEfrom increasing.

Embodiment 2

[Additional Problems]

When the ZC sequence selection method described in Embodiment 1 isapplied as is to a case where a transmission bandwidth is 5 RBs or less,there is a problem that CoMP_UE ZC sequences that will satisfy desiredconditions cannot be secured sufficiently.

For example, when the transmission bandwidth is 3 RBs (M^(RS)=36subcarriers), since ZC sequence length N_(ZC) ^(RS) is a maximum primenumber in units of the numbers of subcarriers smaller than transmissionbandwidth M^(RS), N_(ZC) ^(RS)=31. There are (N_(ZC) ^(RS)−1) ZCsequences with ZC sequence numbers q=1 to (N_(ZC) ^(RS)−1). In thiscase, it is therefore not possible to select any sequence number whichis closest to sequence selection reference value B_(u)′ of equation 4and different from the non-CoMP_UE ZC sequence as the CoMP_UE ZCsequence.

When the transmission bandwidth is 4 RBs and 5 RBs, ZC sequence lengthN_(ZC) ^(RS) becomes 47 and 59, respectively. Thus, it is not possibleto select sequence numbers different from non-CoMP_UE ZC sequences of 30sequence groups one by one as CoMP_UE ZC sequences of 30 sequence groupsin this case, either.

FIGS. 15A and 15B illustrate results of cases where the ZC sequenceselection method of Embodiment 1 is applied to 3 RBs to 6 RBs. FIG. 15Aillustrates each ZC sequence number q assigned to each non-CoMP_UE ZCsequence group (u=0 to 29) calculated based on the sequence selectionreference values of equation 3 (where, the transmission bandwidth is 3to 6 RBs). On the other hand, FIG. 15B illustrates ZC sequence number q′assigned to each CoMP_UE ZC sequence group (u=0 to 29) calculated withthe sequence selection reference values in equation 4 (where, thetransmission bandwidth is 3 to 6 RBs).

It can be confirmed in FIG. 15B that ZC sequence numbers in the shadedarea are the same for non-CoMP_UE and CoMP_UE. Using ZC sequences whoseZC sequence length and ZC sequence number are the same for non-CoMP_UEand CoMP_UE in this way may cause inter-cell interference of DMRS toincrease between non-CoMP_UE and CoMP_UE.

[Configurations of Terminal 100 and Base Station 200]

The configuration of terminal 100 according to Embodiment 2 issubstantially the same as that of Embodiment 1 and is only different inoperations of sequence calculation section 112 and reference signalgenerating section 113 in a case where the transmission bandwidth isequal to or below a predetermined value (e.g., 5 RBs or less). As in thecase of terminal 100, the configuration of base station 200 ofEmbodiment 2 is only different in the operations of the sequencecalculation section and the reference signal generating section includedin reference signal setting section 214 in a case where the transmissionbandwidth is equal to or below a predetermined value (e.g., 5 RBs orless).

[Operation of Embodiment 2]

Here, the operation of sequence calculation section 112 and referencesignal generating section 113 of terminal 100 will be described.Reference signal setting section 214 of base station 200 performssubstantially the same processing, and therefore description thereofwill be omitted.

First, sequence calculation section 112 determines whether thetransmission bandwidth of DMRS (=M^(RS) [subcarriers]) indicated frombase station 200 is equal to or below a predetermined value (e.g., 5 RBsor less) and whether the output from CoMP determining section 111 isCoMP_UE or not. When the determination result is affirmative, sequencecalculation section 112 sets a value greater than transmission bandwidthM^(RS) as ZC sequence length N_(ZC) ^(RS).

Here, sequence calculation section 112 sets, for example, a maximumprime number which does not exceed twice the transmission bandwidth asZC sequence length N_(ZC) ^(RS). That is, in the cases of 3, 4 and 5RBs, ZC sequence length N_(ZC) ^(RS) is assumed to be 71, 89 and 113,respectively. This makes it possible to increase the number of ZCsequences (=N_(ZC) ^(RS)−1) that can be generated.

Next, sequence calculation section 112 calculates CoMP_UE ZC sequencenumber q′ using equation 5-2 based on CoMP_UE ZC sequence length N_(ZC)^(RS) calculated as described above and CoMP_UE sequence selectionreference value B_(u)′ (e.g., equation 4) shown in Embodiment 1.

FIG. 16 is a table illustrating ZC sequence selection results ofEmbodiment 2 in the cases of 3 RBs to 6 RBs. The table in FIG. 16 showsZC sequence length N_(ZC) ^(RS) calculated when the transmissionbandwidth is 3 to 6 RBs and ZC sequence number q′ assigned to eachCoMP_UE ZC sequence group (u=0 to 29).

With ZC sequence number q′ calculated as shown in FIG. 16, compared tothe non-CoMP_UE sequence group in FIG. 15A, it is possible to makedifferent a sequence determination value (ratio between a ZC sequencelength and a ZC sequence number) in any sequence group. Thus, using thisZC sequence number q′ makes it possible to avoid increases in inter-cellinterference of DMRS used for non-CoMP_UE and CoMP_UE.

Note that setting values of the ZC sequence length set by sequencecalculation section 112 with a transmission bandwidth equal to or belowa predetermined value are not limited to the case where they arecalculated from twice the transmission bandwidth. If the setting valueof the ZC sequence length is assumed to be a prime number twice or morethan the transmission bandwidth, it is possible to select one or moresequence numbers closest to sequence selection reference value B_(u)′ inequation 4 and different from non-CoMP_UE ZC sequences as CoMP_UE ZCsequences. Therefore, sequence calculation section 112 and referencesignal generating section 113 may be configured to set a ZC sequencelength corresponding, for example, a system bandwidth and use a ZCsequence, part of which is removed.

Reference signal generating section 113 deletes part of the ZC sequencehaving ZC sequence length N_(ZC) ^(RS), generates a reference signalsequence having a length of transmission bandwidth M^(RS) and outputsthe reference signal sequence to mapping section 102.

FIG. 17 illustrates a first example of truncation processing on a ZCsequence according to Embodiment 2 and FIG. 18 illustrates a secondexample of truncation processing on a ZC sequence according toEmbodiment 2.

Reference signal generating section 113 deletes part of the ZC sequencetaking advantage of symmetry of the waveform thereof as follows. Twoexamples will be described below. Since the waveform of the ZC sequencehas symmetry (waveform, the left and right sides of which are symmetricwith respect to the central axis), the first example is a method thatdeletes part of the ZC sequence so as to maintain this symmetry. Thismakes it possible to avoid increases in cross-correlation (seeWO2009/041066). To be more specific, as shown in FIG. 17, symmetry ofthe waveform can be maintained by deleting the central part of thewaveform of the ZC sequence (waveform shown in the upper row in FIG. 17assuming sequence element #k) and merging the left and right waveforms.

The second example is a method that deletes the first half or last halfof the waveform of the ZC sequence as shown in FIG. 18. Since the ZCsequence has symmetric nature, the first half and the last half of thewaveform have fluctuations at the same level, and it is thereby possibleto avoid increases in PAPR (Peak to Average Power Ratio) and CM (CubicMetric).

When the transmission bandwidth of DMRS M^(RS) [subcarriers]) indicatedfrom base station 200 is greater than a predetermined value (e.g., 6 RBsor more) or when the output from the CoMP determining section isnon-CoMP_UE, the operation is similar to that of Embodiment 1. That is,sequence calculation section 112 and reference signal generating section113 perform the same operation as that of Embodiment 1 in this case.

As described above, according to Embodiment 2, even when thetransmission bandwidth is narrow, it is possible to assign a pluralityof sequence groups which are closest to CoMP_UE sequence selectionreference values B_(u)′ and sequence determination value q/N_(ZC) ^(RS)of which is different from the non-CoMP_UE value. Thus, Embodiment 2 aswell as Embodiment 1 can avoid increases in inter-cell interference ofDMRS used for CoMP_UE and non-CoMP_UE. Furthermore, Embodiment 2 deletespart of the ZC sequence taking advantage of symmetry of the waveform ofthe ZC sequence to generate DMRS, and can thereby avoidcross-correlation of DMRS used for CoMP_UE and increases in PAPR or CM.

Embodiment 3

[Other Problems]

As described in PTL 1, while a cross-correlation between ZC sequenceshaving similar ratios (sequence determination values) between a ZCsequence number and a ZC sequence length is highest, a cross-correlationbetween ZC sequences whose difference in sequence determination valuesis close to 0.5 is second highest. According to the ZC sequenceselection method of Embodiment 1, for example, sequence selectionreference value B₀ corresponding to non-CoMP_UE sequence group numberu=1 becomes B₀=2/31 from equation 3. On the other hand, sequenceselection reference value B₁₇′ corresponding to CoMP_UE sequence groupnumber u=17 becomes B₁₇′=35/62 from equation 4, and the relationshipbetween the two becomes B₁₇′=B₀+1/2. Therefore, when ZC sequences withtwo sequence determination values close to these sequence selectionreference values B₀ and B₁₇′ are selected, the cross-correlation betweenthese ZC sequences increases. When these ZC sequences are used in aneighboring cell in the same frequency band, inter-cell interference ofDMRS gradually increases. Embodiment 3 is intended to avoid suchincreases in inter-cell interference.

[Configurations of Terminal 100 and Base Station 200]

The configurations of terminal 100 and base station 200 according toEmbodiment 3 are substantially the same as those of Embodiment 1 andonly operations of sequence calculation section 112 and the sequencecalculation section of base station 200 are different.

[Operation of Embodiment 3]

Next, the operation of sequence calculation section 112 of terminal 100will be described. Since the sequence calculation section of basestation 200 performs substantially the same operation, and thereforedescription thereof will be omitted.

Sequence calculation section 112 of Embodiment 3 is different fromEmbodiment 1 in the method of setting CoMP_UE UE-specific sequence groupnumber u.

Upon receiving indication of CoMP_UE UE-specific sequence group number ufrom base station 200, sequence calculation section 112 uses thatsequence group number u.

On the other hand, upon receiving no implicit or explicit indication ofUE-specific sequence group number u from base station 200, sequencecalculation section 112 derives CoMP_UE UE-specific sequence groupnumber u from the non-CoMP_UE sequence group number according topredetermined rules. Two specific derivation methods will be describedbelow.

FIG. 19 illustrates a first example of the method of assigning a CoMP_UEsequence group according to Embodiment 3.

The first method is a method whereby UE-specific sequence group number uis selected so that CoMP_UE sequence selection reference value B_(u)′becomes closest to non-CoMP_UE sequence selection reference value B_(u).

In this method, for example, when the number of CoMP_UE sequence groupsis 30 (the same as the number of non-CoMP_UE sequence groups, the numberof divisions of sequence selection reference value Y=1), CoMP_UEUE-specific sequence group numbers may be calculated as shown inequation 7. It is thereby possible to separate cells using sequencegroups having a slightly high cross-correlation corresponding to adifference in sequence selection references of 0.5 as shown in FIG. 19(e.g., non-CoMP_UE sequence group number #1 and CoMP_UE sequence groupnumber #17). According to this method, it is possible to reduceinterference between DMRSs used for CoMP_UE and non-CoMP_UE throughdistance attenuation.[6]CoMP_UE sequence group number=Non-CoMP_UE sequence group number  (Equation 7)

The following effects can be achieved by applying the above-describedmethod of deriving UE-specific sequence group number u. For example, asshown in FIG. 19, let us suppose a case where a CoMP_UE sequence groupnumber is assigned to CoMP_UE located in the vicinity of a boundarybetween two cells to which non-CoMP_UE sequence group numbers #1 and #3are assigned. In this case, when CoMP_UE belongs to a cell to whichnon-CoMP_UE sequence group number #1 is assigned (when the serving cellis #1), the base station need not notify this CoMP_UE of the CoMP_UEsequence group number. For this CoMP_UE, a sequence group number may bederived using the above-described method. Furthermore, let us supposethat this CoMP_UE has moved to a cell to which non-CoMP_UE sequencegroup number #3 is assigned (when the serving cell is #3). The basestation needs to indicate CoMP_UE sequence group number #17 to CoMP_UEonly in this case. Thus, indication of a CoMP_UE sequence group numbercan be omitted in one of two cells.

FIG. 20 illustrates a second example of the method of assigning CoMP_UEsequence groups according to Embodiment 3.

The second method is a method whereby UE-specific sequence group numberu is selected so that CoMP_UE sequence selection reference value B_(u)′becomes closest to non-CoMP_UE sequence selection reference valueB_(u)+0.5.

According to this method, for example, when the number of CoMP_UEsequence groups is 30 (the same as the number of non-CoMP_UE sequencegroups, the number of divisions of the sequence selection referencevalue Y=1), the CoMP_UE UE-specific sequence group number may becalculated as shown in equation 8. Thus, as shown in FIG. 20, sequencegroups having a slightly high cross-correlation corresponding to adifference in sequence selection references of 0.5 (e.g., non-CoMP_UEsequence group number #1 and CoMP_UE sequence group number #17) can beused in cells in the same CoMP reception area. Since one schedulerallocates resources in a cell in the CoMP reception area, the schedulercan assign UEs whose interference increases to different frequencybands. Thus, it is possible to reduce interference of DMRS used forCoMP_UE and non-CoMP_UE by allocating appropriate resources.[7]CoMP_UE sequence group number=(Non-CoMP sequence groupnumber+16)mod(30)  (Equation 8)

Even in a case where such a method of deriving UE-specific sequencegroup number u is applied, if CoMP_UE is located in the vicinity of aboundary between two cells as shown in FIG. 20, an effect can beachieved that indication of CoMP_UE sequence group numbers can beomitted for one of the two. That is, the base station needs to indicate,only to CoMP_UE whose non-CoMP_UE sequence group number is #3 (UE whoseserving cell is #1), the CoMP_UE sequence group number. It is possibleto omit indication of CoMP_UE sequence group numbers to CoMP_UE (UEwhose serving cell is #1) whose non-CoMP_UE sequence group number is #1.

As described above, according to Embodiment 3, it is possible to reduceinterference between DMRSs used for CoMP_UE and non-CoMP_UE by takinginto account the assignment of sequence groups having a slightly highcross-correlation between CoMP_UE and non-CoMP_UE corresponding to adifference in sequence determination value of 0.5.

Moreover, according to Embodiment 3, it is possible to reduce the amountof signaling of UE-specific sequence group numbers by determiningbeforehand the method of deriving CoMP_UE sequence group numbers whenthere is no notification thereof.

The embodiments of the present invention have been described so far.

Although in the above-described embodiments, a terminal whosetransmission signals are received and combined by a plurality of cellsin cooperation is represented by CoMP_UE, CoMP_UE may be read as a UE inwhich a UE-specific DMRS sequence is settable or set. Alternatively,CoMP_UE may be read as a UE that supports 3GPP Rel.11 or later or a UEin which Virtual_cell_ID is settable or set. In addition, CoMP_UE may beread as a UE to which a base station has explicitly indicated that it isCoMP_UE or a UE instructed to use a ZC sequence which is different froma sequence of cell-specific assignment for DMRS.

CoMP_UE may also be represented by a UE which supports Rel.11 or laterand which is connected to a low transmission power base station (picocell, RRH (Remote Radio Head) or the like).

Here, a case will be described where the present invention is applied toa system in which a low transmission power base station is disposed.

FIG. 21 illustrates an example in which sequence group assignmentaccording to the present invention is applied to a system provided witha low power base station.

For example, in LTE Rel.11 or later, as shown in FIG. 21, a macro cellarea in which sequence groups defined in LTE Rel.10 are used may becovered with a plurality of newly arranged low transmission power basestations. When such a plurality of low transmission power base stationsare deployed, the cell radius of each base station becomes smaller, anda reuse distance of sequence groups (distance between cells using thesame sequence group) becomes smaller, resulting in a similar problemthat inter-cell interference increases.

Thus, by assigning a new sequence group described in the above-describedembodiments as a CoMP_UE sequence group to a newly installed lowtransmission power base station, it is possible to achieve effectssimilar to those of the above-described embodiments. That is, thepresent embodiment is applied by assuming a UE to be connected to thenewly installed low transmission power base station as a CoMP_UE and UEsto be connected to other Cells as non-CoMP_UEs according to LTE Rel.11or later. More specifically, as shown in FIG. 21, a case is assumedwhere a sequence group number used by a UE (non-CoMP_UE) connected to amacro cell is #1 and five small cells (also referred to as “pico cells”)of LTE Rel.11 or later are deployed in the cell area. In this case, fivesequence groups are generated using five CoMP_UE sequence selectionreference values B_(u, y)′(B_(u,0)′ to B_(u,4)′ calculated by equation9) by assuming Y=5 in equation 6 and assigned to the respective smallcells. This makes it possible to reduce interference between DMRSs ofUEs connected to different cells (between small cells or between smallcell and macro cell).

$\begin{matrix}\left( {{Equation}\mspace{14mu} 9} \right) & \; \\{B_{u,y}^{\prime} = {\frac{u}{31} + {\frac{y + 1}{31 \cdot 6}\left( {{u = 0},\ldots\mspace{14mu},{29;{y = 0}},\ldots\mspace{14mu},4} \right)}}} & \lbrack 8\rbrack\end{matrix}$

The above-described embodiments have described that CoMP_UE is indicatedfrom a base station to a terminal, and more specifically, UL Grant(UpLink Grant) of PDCCH (Physical Downlink Control Channel) can beapplied as resources and control information used for this indication.In addition, RRC (Radio Resource Control) signaling (higher layersignaling) is also applicable as control information used for thisindication.

Although a case has been described in the above-described embodiments asan example of configuration in which reference signal sequencesaccording to the present invention are applied to DMRS, a configurationmay also be adopted in which reference signal sequences according to thepresent invention are applied to SRS (Sounding Reference Signal).

Moreover, the above-described embodiments have mainly described theconfiguration in which the number of ZC sequences usable for each RB ofa CoMP_UE sequence group is equal to the number of non-CoMP_UE sequencegroups. However, the number of ZC sequences usable for each RB of aCoMP_UE sequence group may be different from the number of non-CoMP_UEsequence groups. For example, as shown in FIG. 1, the number ofnon-CoMP_UE sequence groups is 1 sequence for 3 to 5 RBs and 2 sequencesfor 6 RBs or more. On the other hand, the number of CoMP_UE sequencegroups may be 1 sequence for all RBs as shown in FIG. 22. FIG. 22 is atable illustrating a variation of the method of assigning CoMP_UE ZCsequences.

Narrowing down CoMP_UE ZC sequences to 1 sequence for each RB in thisway can achieve the following effects. That is, DMRS of CoMP_UE requireshigher channel estimation accuracy for MU-MIMO separation. Thus,narrowing down CoMP_UE ZC sequences to 1 sequence for each RB makes itpossible to use only ZC sequences with a sequence determination value(ratio between a ZC sequence number and ZC sequence length) closer todesired sequence selection reference S_(n)′ as DMRS. This reducesinter-cell interference of DMRS, and can thereby improve channelestimation accuracy.

[Exception Handling]

As exception handling, the communication system according to theabove-described embodiment may be configured to allow CoMP_UE toselectively use DMRS of a non-CoMP_UE sequence group and DMRS of aCoMP_UE sequence group in accordance with the situation.

For example, DMRS for RACH message 3 (RACH (Random Access Channel)response) may use non-CoMP_UE ZC sequences regardless of CoMP_UE ornon-CoMP_UE. The base station cannot distinguish whether or not aterminal that transmits RACH message 3 is a UE in which cell-specificsequences can be set. Thus, by adopting such exception handling, thebase station can receive RACH message 3 of CoMP_UE correctly.

When a sequence group number is explicitly indicated by RRC (RadioResource Control) signaling, CoMP_UE may be configured so as to usenon-CoMP_UE ZC sequences for a predetermined period immediately aftersignaling. The base station cannot determinately recognize whether ornot the terminal is updated based on the signaling content from the basestation for the predetermined period after signaling. Thus, by adoptingsuch exception handling, the base station can receive an uplink signalcorrectly even for an indeterminate period immediately after signaling.

In each embodiment described above, the present invention is configuredwith hardware by way of example, but the invention may also be providedby software in concert with hardware.

In addition, the functional blocks used in the descriptions of theembodiments are typically implemented as LSI devices, which areintegrated circuits. The functional blocks may be formed as individualchips, or a part or all of the functional blocks may be integrated intoa single chip. The term “LSI” is used herein, but the terms “IC,”“system LSI,” “super LSI” or “ultra LSI” may be used as well dependingon the level of integration.

In addition, the circuit integration is not limited to LSI and may beachieved by dedicated circuitry or a general-purpose processor otherthan an LSI. After fabrication of LSI, a field programmable gate array(FPGA), which is programmable, or a reconfigurable processor whichallows reconfiguration of connections and settings of circuit cells inLSI may be used.

Should a circuit integration technology replacing LSI appear as a resultof advancements in semiconductor technology or other technologiesderived from the technology, the functional blocks could be integratedusing such a technology. Another possibility is the application ofbiotechnology, for example.

The disclosure of the specification, drawings, and abstract included inJapanese Patent Application No. 2012-052854 filed on Mar. 9, 2012 isincorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The present invention is useful in mobile communication systems.

REFERENCE SIGNS LIST

-   100 Terminal-   101 Antenna-   102 Mapping section-   103 IFFT section-   104 CP adding section-   105 Transmitting section-   106 Receiving section-   107 Demodulation section-   108 Transmission processing section-   109 Reception processing section-   110 Reference signal processing section-   111 CoMP determining section-   112 Sequence calculation section-   113 Reference signal generating section-   200 Base station-   201 Antenna-   202 Receiving section-   203 CP removing section-   204 Demultiplexing section-   205, 208 FFT section-   206, 209 Demapping section-   207 Frequency domain equalization section-   210 CH estimation section-   211 IDFT section-   212 Demodulation section-   213 Decoding section-   214 Reference signal setting section-   215 Modulation section-   216 Transmitting section-   217 Transmission processing section-   218 Reception processing section

The invention claimed is:
 1. A terminal apparatus comprising: generating circuitry, which, in operation: selects, when coordinated-reception by a plurality of base stations is not applied to the terminal apparatus, a first reference signal sequence, determined from a first selection reference value, among a plurality of reference signal sequences, as a non-coordinated-reception reference signal sequence, the first selection reference value corresponding to a first sequence group number of a first sequence group assigned to a cell to which the terminal apparatus belongs among a plurality of selection reference values, the first selection reference value being a ratio between a value corresponding to the first sequence group number and a sequence length of a reference signal sequence of a minimum transmission bandwidth in the first sequence group; selects, when coordinated-reception by the plurality of base stations is applied to the terminal apparatus, a second reference signal sequence, determined from one or a plurality of second selection reference values different from the first selection reference value, among the plurality of reference signal sequences, as a coordinated-reception reference signal sequence different from the non-coordinated-reception reference signal sequence, the second selection reference values being values set between two selection reference values which are adjacent to each other, the second selection reference values corresponding to a second sequence group number of a second sequence group assigned specifically to the terminal apparatus; and generates a reference signal based on the selected reference signal sequence; and a transmitter coupled to the generating circuitry and operation, transmits the generated reference signal.
 2. The terminal apparatus according to claim 1, wherein: when coordinated-reception by the plurality of base stations is not applied to the terminal apparatus, the generating circuitry selects the first reference signal sequence of sequence number q that makes an absolute value of differences between Bu and q/N lowest, where Bu represents the plurality of selection reference values, N represents a sequence length of a reference signal sequence corresponding to a transmission bandwidth, and q represents a sequence number to identify a plurality of types of reference signal sequences of an identical sequence length; and when coordinated-reception by the plurality of base stations is applied to the terminal apparatus, the generating circuitry selects the second reference signal sequence of sequence number q′ that makes an absolute value of differences between Bu′ and q′/N lowest, where Bu′ represents the second selection reference values, N represents a sequence length of a reference signal sequence corresponding to a transmission bandwidth, and q′ represents a sequence number to identify a plurality of types of reference signal sequences of an identical sequence length.
 3. The terminal apparatus according to claim 1, wherein Y (Y is a natural number) second selection reference values are set between the two first selection reference values, which are adjacent to each other, at uniform intervals.
 4. The terminal apparatus according to claim 1, wherein, when coordinated-reception by the plurality of base stations is applied to the terminal apparatus, the generating circuitry selects, when a transmission bandwidth is narrower than a predetermined bandwidth, the second reference signal sequence of a sequence length corresponding to a bandwidth longer than the transmission bandwidth as the coordinated-reception reference signal sequence, and generates the reference signal by deleting part of the selected second reference signal sequence.
 5. The terminal apparatus according to claim 1, wherein, when coordinated-reception by the plurality of base stations is applied to the terminal apparatus, the generating circuitry selects, when the second sequence group number is not indicated from the base station, the second selection reference value closest to the first selection reference value corresponding to the first sequence group number.
 6. The terminal apparatus according to claim 1, wherein, when coordinated-reception by the plurality of base stations is applied to the terminal apparatus, the generating circuitry selects, when the second sequence group number is not indicated from the base station, the second selection reference value whose absolute value of a difference from the first selection reference value corresponding to the first sequence group number is closest to 0.5.
 7. The terminal apparatus according to claim 1, wherein the reference signal sequence is a Zadoff-Chu sequence.
 8. The terminal apparatus according to claim 1, wherein the reference signal is an uplink demodulation reference signal.
 9. The terminal apparatus according to claim 1, comprising determining circuitry which, in operation, determines whether the coordinated-reception is applied to the terminal apparatus based on whether a terminal-specific reference signal sequence is set by the base station.
 10. The terminal apparatus according to claim 1, wherein the sequence length of the reference signal sequence of the minimum transmission bandwidth in the first sequence group is a maximum prime numerical value that does not exceed a number of subcarriers included in the minimum transmission bandwidth.
 11. The terminal apparatus according to claim 1, wherein, when the value corresponding to each sequence group number u is u+1 and the sequence length of the reference signal sequence of the minimum transmission bandwidth in the first, sequence group is 31, the plurality of selection reference values are (u+1)/31.
 12. A radio transmission method comprising: selecting, when coordinated-reception by a plurality of base stations is not applied to a terminal apparatus, a first reference signal sequence, determined from a first selection reference value, among a plurality of reference signal sequences, as a non-coordinated-reception reference signal sequence, the first selection reference value corresponding to a first sequence group number assigned to a cell to which the terminal apparatus belongs among a plurality of selection reference values, the first selection reference value being a ratio between a value corresponding to the first sequence group number and a sequence length of a reference signal sequence of a minimum transmission bandwidth in the first sequence group; selecting, when coordinated-reception by the plurality of base stations is applied to the terminal apparatus, a second reference signal sequence, determined from one or a plurality of second selection reference values different from the first selection reference value, among the plurality of reference signal sequences, as a coordinated-reception reference signal different from the non-coordinated-reception reference signal sequence, the second selection reference values being values set between two selection reference values which are adjacent to each other, the second selection reference values corresponding to a second sequence group number of a second sequence group assigned specifically to the terminal apparatus; generating a reference signal based on the selected reference signal sequence; and transmitting the generated reference signal.
 13. A base station apparatus comprising: setting circuit which, in operation, selects, when coordinated-reception by a plurality of base stations is not applied to a terminal apparatus, a first reference signal sequence, determined from a first selection reference value, among a plurality of reference signal sequences, as a non-coordinated-reception signal sequence, the first selection reference value corresponding to a first sequence group number of a first sequence group assigned to a cell to which the terminal apparatus belongs among a plurality of selection reference values, the first selection reference value being a ratio between a value corresponding to the first sequence group number and a sequence length of a reference signal sequence of a minimum transmission bandwidth in the first sequence group; and selects, when the coordinated-reception by the plurality of base stations is applied to the terminal apparatus, a second reference signal sequence, determined from one or a plurality of second selection reference values different from the first selection reference value, among the plurality of reference signal sequences, as a coordinated-reception reference signal sequence different from the non-coordinated reference signal sequence, the second selection reference values being values set between two selection reference values which are adjacent to each other, the second selection reference values corresponding to a second sequence group number assigned specifically to the terminal apparatus; a receiver which, in operation, receives a signal transmitted from the terminal apparatus; and estimating circuitry, coupled to the setting circuitry and the receiver, and which, in operation, performs channel estimation based on (i) the received signal and (ii) the first reference signal sequence or the second reference signal sequence selected by the setting circuitry.
 14. A channel estimation method comprising: selecting, when coordinated-reception by a plurality of base stations is not applied to a terminal apparatus, a first reference signal sequence, determined from a first selection reference value, among a plurality of reference signal sequences, as a non-coordinating-reception reference signal sequence, the first selection reference value corresponding to a first sequence group number of a first sequence group assigned to a cell to which the terminal apparatus belongs among a plurality of selection reference values the first selection reference value being a ratio between a value corresponding to the first sequence group number and a sequence length of a reference signal sequence of a minimum transmission bandwidth in the first sequence group; selecting, when coordinated-reception by the plurality of base stations is applied to the terminal apparatus, a second reference signal sequence, determined from one or a plurality of second selection reference values different from the first selection reference value, among the plurality of reference signal sequences, as a coordinated-reception reference signal sequence different from the non-coordinated-reception reference signal sequence, the second selection reference values being values set between two selection reference values which are adjacent to each other, the second selection reference values corresponding to a second sequence group number of a second sequence group assigned specifically to the terminal apparatus; receiving a signal transmitted from the terminal apparatus; and performing channel estimation based on (i) the received signal and (ii) the selected first reference signal sequence or the selected second reference signal sequence. 